Three-dimensional imaging of active structures using earthquake aftershocks: the Northridge thrust, California

Aftershocks of the 1994 Northridge earthquake (M 6.8) allow us to image the structure beneath the San Fernando Valley in northwestern Los Angeles in three dimensions. We combine aftershocks and geological data to build an image of the three-dimensional (3-D) geometry of the previously unrecognized, north-vergent Northridge blind thrust to a depth of 21 km. The most striking feature of the imaged fault is mega-corrugations oriented parallel to the mean aftershock slip vector, with most of the 1994 slip confined to west of the largest corrugation. We also image the partially overlying south-vergent San Fernando thrust, which broke to the surface in a complex rupture in 1971 (M 7.1). Both thrusts produce fault-related folding because of either fault propagation or fault bends; however, this deeper folding is masked by overlying complex deformation in the cover, which is one reason why the Northridge thrust was not identified until it ruptured in 1994. We use trishear fold modeling based on our 3-D fault geometry to evaluate possible folding due to slip on the Northridge thrust as well as its interaction with the overlapping San Fernando thrust and with shallow structures in the cover. This example illustrates the importance of earthquake data to structural geology and the value of its 3-D integration with surface and near-surface geological data.